Elevator car doors have historically been driven by rotary motors through complex linkages. For center-opening double doors, the complexity of the linkage is even greater. Additionally, rotary door openers are difficult to install and require frequent, costly maintenance.
Some more recent elevator doors employ linear motors wherein the force created between the primary and the secondary is applied directly to the elevator doors to produce corresponding linear motion. While linear door operators are more reliable, requiring little if any maintenance, they can be expensive. For double door configurations, options include having a motor including primary, secondary, backiron and drive system for each of the door panels; but this is doubly expensive. Another option is to use a single motor to move one panel and relate the other panel to it, with a cable or the like; but such a system is far more difficult to control. Another option is to place the primary on one door panel and the secondary on the other door panel; this requires both parts of the motor to move, increasing the moving mass in the door system and requiring a moving cable to the motor winding. Two-speed door sets require one motor mounted on the slow door to drive the fast door and one motor mounted on the cab (or on the slow door) to drive the slow door, or single motors using relating cables.